A Closer Look at the Nature of Lighting

Understanding the basics of lighting, including color, intensity, and illuminance helps you choose the right solution for multiple applications.The human eye receives a small range of frequencies of electromagnetic radiation called light. Visible light frequencies and matching colors range from violet to red. We base the frequency of light radiation on the number of times per second the light energy

John Pascal, EC&M Books Editor | Dec 01, 1999

Understanding the basics of lighting, including color, intensity, and illuminance helps you choose the right solution for multiple applications.

The human eye receives a small range of frequencies of electromagnetic radiation called light. Visible light frequencies and matching colors range from violet to red. We base the frequency of light radiation on the number of times per second the light energy wave makes a complete alternating cycle (cycles per second) or in modern terms (Hertz). Note: Frequency relates to the length of the light energy wave, or to its wavelength, as:

Frequency (Hz) = 1 / wavelength (meters)

So, as the frequency increases, the wavelength becomes shorter.

Color. A light source emits one or more frequencies of light wave radiation. It may emit a continuous spectrum (all visible wavelengths) or only a "band" spectrum, containing only one or a few wavelengths. The sun and tungsten-filament lamps each emit a continuous spectrum seen as "white" by the human eye. However, color combinations of individual frequencies of light combine to trick the eye into "seeing" colors that are not actually presented to the eye.

Not every light source delivers energy in a continuous spectral distribution. Light sources such as strobe lights, high-intensity discharge lamps, and fluorescent lamps deliver light energy at only one or a few specific frequencies or colors. If a light source emits only a "band" spectrum, then not every wavelength of light is available to reflect from objects. Therefore, the light energy that finally gets to the eye from this type of light source tends to deliver false color information. You must consider this when designing lighting systems.

To add colors to arc-type lamp outputs, lamp manufacturers coat the inner glass lamp surface with specific mixtures of phosphors that create light outputs seen by the human eye as certain colors. Each phosphor emits its own unique wavelength, and these all combine to form a new color perceived by the eye. For example, red and yellow combine within the eye to form orange.

Color temperature, measured in degrees Kelvin, describes the overall "warmth" or "coolness" produced by the light source. A lower color temperature indicates a "warm" source that emphasizes reds, oranges, and yellows. A higher color temperature designates a "cool" source that emphasizes blues and greens. A Color Rendering Index (CRI), measured in percent of normal (with "normal" being 100%), shows how normal and natural a specific light source makes objects appear. The higher the CRI, the better it makes people and objects look.

Reflected and absorbed light also influence what the eye distinguishes. Everything illuminated by a lamp absorbs certain amounts of each frequency of light falling upon it and reflects others. Our brains interpret the unique combination of reflected light that finally reaches the eye as a color. Where color is important, the lighting designer must provide a light source that emits the colors of light that the object reflects, or else you won't see the object correctly.

Intensity and illuminance. We define one burning candle as emitting light in a given direction at an intensity of one "candlepower." A measure of the illumination level produced by that candle is the "lumen." A measure of the illuminance on a curved surface 1 ft from the candle resulting from the lumens emitted by that candle is the "footcandle." That is, one lumen is the quantity of light that falls on a 1-sq-ft area whose surface is equidistant at 1 ft from a light source whose intensity is one candlepower.

When luminous flux or light falls upon a surface, we consider that surface illuminated. The unit for measuring illuminance is the "footcandle," where one footcandle equals one lumen per sq ft.

The candlepower, also called the "candela," is the unit of luminous intensity of light emitted by a light source in a given direction. It also describes the light within a 3-D angle, assuming a point source of light. (When the light travels away from the source, the 3-D angle covers a larger and larger area, while the angle remains the same). Thus, the same luminous flux is spread over a larger surface as the distance from the light source to the lighted surface increases. Although strict mathematics require a multitude of calculations for a flat surface (since the distances from the light source to the different points on the flat surface are all different), the simplification of using the average distance to a flat surface produces sufficiently accurate values.

Lamp catalogs include both initial and average (over lamp life) data about the quantity of lumens each type of lamp emits. When you insert lamps into lighting fixtures, the luminaries absorb some of the lamp lumens. Dirt buildup also affects light output levels. Basic lighting calculations involve simply adding together the average lamp lumen outputs from all of the lamps in an area, subtracting the losses to dirt and absorption within the fixtures, and dividing the remaining lumens by the area in sq ft.

Outdoors, where there are no reflective surfaces, only 50% of the lamp lumens ultimately reach the designated surface. Therefore, a "quick" method of approximating illuminance on an outdoor surface is to ignore color; sum the mean lumen outputs of all lamps, and multiply it by 50%; and divide the resulting number by the area in sq ft. These steps result in approximate lumens/sq ft, or footcandles, for the area.